1. Field of the Invention
The present invention relates to a mooring system for anchoring sea going ships, such as storage or production tanker vessels, and more particularly, relates to a submerged single point mooring system fixed to the seabed and which is particularly adapted for use in arctic waters having surface hazards such as pack ice or icebergs.
Various types of mooring systems have been developed to facilitate the loading and unloading of large sea going ships, such as super tankers in excess of 200,000 tons, wherein the ships have such deep drafts that natural harbors are frequently unable to accommodate them, and wherein their considerable size causes the mooring forces to reach exceedingly high levels. Accordingly, as an alternate to the construction of new natural harbors for such large ships, with all of the attendant high constructions and maintenance costs, such ships can be moored at sea in relatively deep water and the liquid cargo of the tankers may be transferred to and from the moored ships through the intermediary of underwater pipelines extending between the ship and the shore facilities.
In order to accomplish the foregoing, the various types of mooring systems have been developed so as to be adapted to readily operate under different environmental conditions and taking cognizance of different surface hazards such as ice, rough seas and congested ship traffic. Among such mooring systems there have been developed types of systems which are particularly adapted to counter the hazards present in arctic waters, such as icebergs, pack or field ice, which can cause extensive damage to or even destroy the mooring system components which project to and above the surface of the water.
2. Discussion of the Prior Art
One of the major problems in the mooring of sea going vessels, such as large tankers, in arctic waters subject to extensive ice formations can generally be overcome by constructing the mooring system to be submersible to be able to avoid surface hazards when these are encountered, and in which such mooring systems includes structural components which are elevated above the surface of the arctic or hazardous waters for only limited periods of time.
Among the mooring systems which are designed to provide for the anchoring or mooring of large vessels, such as tankers, in deep waters are the socalled pivoted tower single point mooring systems. Floory et al. U.S. Pat. No. 3,614,869 discloses such a mooring system in which a vertical column forms a support for production or flow lines and risers, and is connected by means of a ball joint to a base which is anchored to or fixed on the seabed, and wherein the upper end of the column supports a pivotable tower above the surface of the water, to which there may be readily connected a vessel, allowing the vessel to weathervane about the mooring in order to conform to different surface and weather conditions. Suitable hoses and pipelines interconnect the tower and the vessel to facilitate loading or unloading of the fluid cargo of this vessel. This type of mooring structure, although generally satisfactory for operation in relatively calm waters, is not readily serviceable in stormy arctic waters wherein extreme ice may damage or even destroy the projecting tower structure of such a mooring system.
Other single point mooring system wherein a portion of the mooring structure extends above the surface of the water are disclosed, respectively, in Tuson U.S. Pat. No. 3,980,037 and Papmahl U.S. Pat. No. 4,254,521. In each of these systems, a vertical column has its lower end pivotally hinged to a base which is fastened on the seabed, and with the upper end of the tower including an extension projecting above the surface of the water to provide for the support of a jib or boom for connecting a vessel, such as a large tanker, to the mooring. In each of these patents, the single point mooring systems are primarily adapted for operation in relatively calm waters, and in waters without surface hazards such as extensive ice commonly encountered in arctic waters which may tend to damage or even destroy the protruding structure of the mooring system.
In order to avoid the danger of damage to a mooring system for sea going ships, such as large sized tankers, which are subjected to surface hazards, such as pack ice which is encountered under arctic weather conditions, submersible tanker mooring systems have been developed wherein the systems are adapted to be operated without any structural components of the mooring systems projecting above the surface of the waters.
Frankel U.S. Pat. No. 3,664,388 discloses a submersible tanker mooring system in which a bell is adapted to be raised or lowered within a tank located in the seabed, to provide for connection to a conduit arrangement communicating with a tanker which is attached to the bell by means of a suitable anchoring cable. During the presence of heavy ice formations in the water, the bell may be lowered so as to be completely submerged beneath the surface of the water, and to thereby permit the ice to float thereover without causing any damage to the mooring structure. However, the mooring system requires a relatively complex hydraulic actuating system for the bell and, moreover, does not permit the bell to be universally articulated relative to the bottom of the seabed so as to render it incapable of absorbing large lateral forces. Additionally, the submersible bell mooring arrangement according to Frankel is of a relatively complex and cumbersome structure requiring extensive hydraulic valving during its operation and does not allow for diverless access to its interior.
Gratz U.S. Pat. No. 3,722,223 discloses a submersible single point mooring facility in which a column structure extending from an anchoring connection with a base fixed to the seabed is adapted to be raised above the surface of the water so as to allow for connection to a tanker or vessel, or submerged to lie flat on the seabed. The structure, although providing for protection of the mooring buoy against accidental impact by passing ships or damage from surface ice, is normally raised above the surface of the water during operation and thus is not designed for extended or protracted use beneath the surface of the arctic waters. Additionally, the structure of this submersible single point mooring facility is rather complex and does not provide for diverless access to the interior of the mooring facility.
Similarly, Lecomte U.S. Pat. No. 3,899,990 discloses a mooring system with submersible column which is pivotally anchored to a base fixed to the seabed, and which may be raised above the surface of the water in order to allow for connection to a vessel on the surface. In this instance, the structure of the system is in a raised position to project above the water surface and is not accessible without the utilization of a diver under normal operating conditions.